Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Sterilisers are used in the medical, food and packaging industries to kill and thereby prevent the transmission of transmissible agents such as spores, fungi, and bacteria. A typical steriliser creates a set of physical conditions in a sterilisation chamber that effectively kills nearly all of these transmissible agents.
Contacting articles in need of sterilisation with sterilant aerosols is one known method of sterilisation. A typical aerosol sterilisation apparatus of the prior art has a sterilisation chamber with inlet and an outlet valves, an aerosol generator (typically a nebuliser) in fluid communication with the chamber via the inlet valve and a fan upstream of, and in fluid communication with, the aerosol generator.
In use, an article requiring sterilisation is placed in the chamber, which is then sealed. The aerosol inlet valve is opened and the outlet valve is closed. The fan is engaged, which creates a gas stream through or the past the aerosol generator, into the chamber. A passive vent in the sterilisation chamber allows for pressure equalization as required, to permit gas flow in and out of the sterilisation chamber. The aerosol generator, which contains the desired sterilant, is then activated, putting a large number of small sterilant droplets into gas stream. The droplets are carried by the gas stream to create an aerosol which travels into the sterilisation chamber. The sterilant concentration in the aerosol stream can be adjusted by changing either the flow rate of the gas stream, the productivity of the aerosol generator, or the concentration of the liquid sterilant used.
The passive waste vent allows some flow to pass through it, allowing the sterilisation chamber to remain at approximately room pressure. This passive system may include a pathway for flow to the outside air past catalytic elements that react with the sterilant and break the sterilant down into a safer chemistry suitable for disposal.
After a period of time, the fan and the aerosol generator are deactivated and the air inlet valve is closed, hence completing the sterilant delivery phase. The exit valve is then opened and aerosol is actively removed, typically by way of a pump that pulls aerosol and vapour out of the sterilisation chamber at a high rate. The removal system may include a pathway for flow between the sterilisation chamber and outside air past catalytic elements that react with the sterilant and break the sterilant down into a safer chemistry suitable for disposal. The passive vent allows a source of fresh air to be drawn into the sterilisation chamber from the outside air.
It is generally desirable for the total sterilisation cycle time to be as short as possible. Short reprocessing durations increases the number of times the sterilised article can be used in a given period, which in turn increases the number of patients per day that can be treated. In the case where the article to be sterilised is a high-cost medical device, short cycle times can generate significant financial savings for a health care provider.
One of the limitations of using an aerosol-based steriliser is that in order to gain the required level of microbiological reduction in a short sterilisation time a high concentration (i.e. a high mist density) of aerosol sterilant is required. During sterilisation, a high concentration of aerosol sterilant causes droplets to coalesce on the surface of the article. This can also lead to multilayer B.E.T.-like absorption on the surface of the sterilized article. Coalesced and absorbed droplets can be difficult to remove from the article at the end of the sterilisation process. Large levels of residual sterilant left on the sterilised article can be harmful to operators and patients and as such are undesirable in a fully automated sterilisation device. Maximum residual levels of sterilant may be defined by the relevant standards, where these exist, or by biocompatibility testing, common usage or other assessment.
While the residual sterilant may be removed by washing, this is an expensive feature to add to an automated sterilisation device, and requires sterile water and fresh water supplies that cannot always be easily obtained. Alternatively, it is also undesirable to have staff hand-washing articles, as this requires the use of safety apparatus which can be expensive (such as fume hoods), can take up valuable time and space and moreover increases the risk of harmful sterilant coming into contact with the operator.
A washing phase also requires a subsequent drying phase which adds considerably to apparatus turn-around times.
Another method for removing residual sterilant is through aeration. Residual removal and evaporation can be achieved by passing a gas stream over the article. Large coalesced droplets take some time to be removed by this process, and this can lead to long cycle times. Higher flow rates or larger suction devices can be used to speed this process, however these devices are often noisy, bulky, and expensive. Devices of this nature are often relegated to central sterilisation areas, adding handling time and effort required to move articles between patients and sterilisation machines, and increasing the total reprocessing time between patients.
It is known in the art that sterilisers often operate using two sterilisation cycles; a first cycle to gain a first 6 log reduction in microbes, followed by a second cycle to achieve a further 6 log reduction in microbes. Aerosol sterilisers that achieve this level of microbial reduction in a single cycle require long sterilisation durations with low sterilant concentrations; long aeration times to remove residual sterilant, or use a powerful waste removal system that is bulky, noisy and/or expensive.
It is desirable to provide an aerosol-based disinfection system that can meet desired microbiological efficacy targets, is fast in total cycle time (including sterilisation and residual removal), is small, low-noise, does not require a fresh water supply, and can be conveniently located close to patients.
As used herein, the term “concentration” is used to refer to the amount or volume of active sterilising agent (such as hydrogen peroxide) relative to the amount or volume of inert carrier fluid (usually water) present. The term can be used in relation to a bulk liquid, to an individual aerosol particle, or to a collective group of aerosol particles generally, although it is not necessary that all particles in an aerosol have the same concentration, for example, if an aerosol arises from two different sources or if an aerosol has been partially modified in space or time.
The term “density” in relation to an aerosol refers to the amount of the total volume that is filled with aerosol particles. The density is a measure of a combination of aerosol droplet volume and the number of aerosol droplets per unit volume. Larger droplets or a higher number of droplets per unit area will both increase aerosol density, whereas smaller droplets or fewer droplets per unit volume will both decrease aerosol density.
The dosage of sterilant delivered is a function of the concentration, the density and the delivery time.